fix: Accept string instance_id in telemetry endpoint

The orchestrator sends instance_id as a string (e.g., "letsbe-orchestrator")
but the endpoint was expecting a UUID path parameter. This caused 422
validation errors when orchestrators tried to send telemetry.

- Changed path parameter from UUID to str
- Lookup instance by Instance.instance_id (string) instead of Instance.id (UUID)
- Store telemetry with instance.id (UUID) for correct FK relationship
- Updated TelemetryPayload schema to use str instead of UUID

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/schemas/telemetry.py

@@ -6,7 +6,6 @@ unknown fields, preventing accidental PII leaks.
 
 from datetime import datetime
 from typing import Optional
-from uuid import UUID
 
 from pydantic import BaseModel, ConfigDict, Field
 
@@ -87,7 +86,7 @@ class TelemetryPayload(BaseModel):
 
     model_config = ConfigDict(extra="forbid")
 
-    instance_id: UUID = Field(..., description="Instance UUID (must match path)")
+    instance_id: str = Field(..., description="Instance ID string (must match path)")
     window_start: datetime = Field(..., description="Start of telemetry window")
     window_end: datetime = Field(..., description="End of telemetry window")
     uptime_seconds: int = Field(ge=0, description="Orchestrator uptime in seconds")